Materials with low thermal expansion coefficients (CTEs) that will survive temperature cycling are of interest in a variety of applications on the microscale, including actuation, precision assembly, and injection mold fabrication. Invar alloys exhibit a low positive coefficient of thermal expansion ranging between 0.2 – 1.2 µm/m-°C at room temperature.

The electrodeposition process for fabricating nanoscale multilayers of Invar and copper in micro-patterns was characterized to assess their value in various MEMS applications. In an ongoing effort to maintain a stable CTE and effectively control the grain growth of Invar, nano-multilayers of near Invar-like FeNiCu and copper were electrodeposited into a pattern of 100µm tall microposts. A harder material than Invar, which could be used in mold insert and sensing applications, was of interest.

Characterization of the FeNiCu electrolyte was done on rotating Hull Cell to determine the exact Invar plating range. EDXRF composition measurements showed that an iron to nickel ratio of Invar composition was obtained for current densities between 52.5 - 56 mA/cm², and copper alone was plated between 0.5 - 1 mA/cm². Microposts were fabricated using the LIGA microfabrication process and 100µm diameter by 100µm tall posts with fixed thickness (12.5nm) FeNiCu layers were deposited alternating with 1 nm, 4 nm, 5 nm, 7 nm, and 9 nm thick copper layers using a two level current density pulse.

The presence of the nanoscale multilayers was confirmed by TEM. Multilayer microposts were tested for their thermal expansion behavior to study the effect of varying the copper layer thickness on the response to heating from ambient to 300°C. As deposited, the multilayer alloy exhibited a negative CTE; for a 2.5 nm thick Cu layer the CTE was -3.28µm/m-°C up to 150°C and more negative for higher temperatures. Two subsequent heating/cooling cycles resulted in the material exhibiting a positive CTE, where the multilayer provided energy for the reconfiguration of the alloy into a more stable, positive CTE form, comparable to bulk Invar.

The average microhardness of the as-deposited multilayer measured 50 on the Rockwell C scale and was comparable with H13 and P20 type tool steels.